In the cleaning of industrial waste gases it is frequently desirable, and in most cases a requirement of the law, that hydrogen sulfide be scrubbed from the combustion gases. The scrubbing of hydrogen sulfide from the combustion gases can be effected with a so-called Stretford scrubbing liquid which generally consists of an aqueous sodium carbonate solution which can include anthroquinone disulfonic acid and sodium vanadate.
In this process, the solubilized or dissolved hydrogen sulfide is oxidized to elemental sulfur which is dispersed, suspended or flotated in the scrubbing liquid.
The sulfur/scrubbing-liquid suspension with which the present invention is concerned thus can be a suspension in the liquid involved in the Stretford process, although the invention is also applicable to sulfur/water suspensions, regardless of how derived, whether from gas treatment or cleaning or otherwise, and containing elemental sulfur dispersed or entrained in finely divided form in an aqueous liquid.
It is known to separate sulfur/liquid suspensions, e.g. a sulfur suspension in the Stretford scrubbing liquid, by raising the temperature of the suspension to a level above the melting point of the sulfur and to introduce the heated suspension into a heated separating vessel which can be under pressure and designed to permit the settled layer of liquid sulfur to be withdrawn from the bottom while the scrubbing liquid, riding above the sulfur layer, is continuously decanted from the vessel.
It should be understood that the feed of the suspension into the separating vessel and the rheological conditions therein must be established in such a way as to enable the continuous decantation of the aqueous phase, i.e. to allow the decantation process to remain undisturbed by turbulence and flow-direction changes.
In the conventional process the suspension, whose sulfur particles have usually already been melted, is continuously forced into the vessel which is completely filled with the liquid so as to be free from any air cushion in a space above the liquid. The separating vessel is a cylindrical upright receptacle and the suspension is generally introduced at a central region thereof with the scrubbing liquid or aqueous phase being withdrawn from the head while the liquid sulfur is withdrawn from and/or accumulates at the base or sump of the vessel.
Naturally, valves are provided in this system for controlling the feed of the suspension, the withdrawal of the aqueous phase and the removal of the sulfur.
In the conventional process, moreover, the highest possible temperature is maintained in the vessel, as is determined by the separating pressure in the vessel.
Experience with this earlier system, however, has shown that it is is not free from disadvantages of which the most significant is the presence of sulfur in the decanted scrubbing liquid. This of course signifies that the separation is incomplete.
Since the scrubbing liquid generally derives from and is recirculated in a plant for the treatment or cleaning of industrial waste gases, stoppages in the duct system between the separating vessel and the gas-cleaning scrubber may result from accumulations of sulfur in the system.
To prevent the entrainment of sulfur with the scrubbing liquid withdrawn from a separating vessel, it is common practice to provide the upper portion of this vessel with flow-directing and direction-changing baffles or like elements, generally of ring configuration.
The effect of these devices is not fully satisfactory since the problem, in large measure, remains and any reduction in the sulfur entrainment appears to be tied to a reduction in the throughput of the separating vessel, this being a significant economic disadvantage.